hanchenye-llvm-project/lld/docs/Readers.rst

.. _Readers:

Developing lld Readers
======================

Introduction
------------

One goal of lld is to be file format independent.  This is done
through a plug-in model for reading object files. The lld::Reader is the base
class for all object file readers.  A Reader follows the factory method pattern.
A Reader instantiates an lld::File object (which is a graph of Atoms) from a
given object file (on disk or in-memory).

Every Reader subclass defines its own "options" class (for instance the mach-o 
Reader defines the class ReaderOptionsMachO).  This options class is the 
one-and-only way to control how the Reader operates when parsing an input file
into an Atom graph.  For instance, you may want the Reader to only accept
certain architectures.  The options class can be instantiated from command
line options, or it can be subclassed and the ivars programmatically set. 


Where to start
--------------

The lld project already has a skeleton of source code for Readers of ELF, COFF,
mach-o, and the lld native object file format.  If your file format is a
variant of one of those, you should modify the existing Reader to support
your variant.  This is done by adding new ivar(s) to the Options class for that
Reader which specifies which file format variant to expect.  And then modifying
the Reader to check those ivars and respond parse the object file accordingly.

If your object file format is not a variant of any existing Reader, you'll need
to create a new Reader subclass. If your file format is called "Foo", you'll 
need to create these files::

    ./include/lld/ReaderWriter/ReaderFoo.h
    ./lib/ReaderWriter/Foo/ReaderFoo.cpp
    
The public interface for you reader is just the ReaderOptions subclass
(e.g.  ReaderOptionsFoo) and the function to create a Reader given the options::

    Reader* createReaderFoo(const ReaderOptionsFoo &options);
    
In the implementation, you can define a ReaderFoo class, but that class is 
private to your ReaderWriter directory.

    
Readers are factories
---------------------

The linker will usually only instantiate your Reader once.  That one Reader will 
have its parseFile() method called many times with different input files.  
To support a multithreaded linking, the Reader may be parsing multiple input 
files in parallel. Therefore, there should be no parsing state in you Reader
object.  Any parsing state should be in ivars of your File subclass or in 
some temporary object.  

The key method to implement in a reader is::

  virtual error_code parseFile(std::unique_ptr<MemoryBuffer> mb,
                               std::vector<std::unique_ptr<File>> &result);

It takes a memory buffer (which contains the contents of the object file
being read) and returns an instantiated lld::File object which is
a collection of Atoms. The result is a vector of File pointers (instead of
simple a File pointer) because some file formats allow multiple object
"files" to be encoded in one file system file.


Memory Ownership
----------------

If parseFile() is successful, it either passes ownership of the MemoryBuffer
to the File object, or it deletes the MemoryBuffer.  The former is done if the
Atoms contain pointers into the MemoryBuffer (e.g. StringRefs for symbols
or ArrayRefs for section content).  If parseFile() fails, the MemoryBuffer
must be deleted by the Reader.

Atoms objects are always owned by their File object.  During core linking 
when Atoms are coalesced or dead stripped away, core linking does not delete
those Atoms. Core linking just removes those unused Atoms from its internal 
list. The destructor of a File object is responsible for deleting all Atoms
it owns, and if ownership of the MemoryBuffer was passed to it, the File  
destructor needs to delete that too.


Making Atoms
------------

The internal model of lld is purely Atom based.  But most object files do not
have an explicit concept of Atoms, instead most have "sections".  The way
to think of this, is that a section is just list of Atoms with common 
attributes. 

The first step in parsing section based object files is to cleave each 
section into a list of Atoms.  The technique may vary by section type.  For
code sections (e.g. .text), there are usually symbols at the start of each 
function. Those symbol address are the points at which the section is cleaved
into discrete Atoms.  Some file formats (like ELF) also include the 
length of each symbol in the symbol table.  Otherwise, the length of each 
Atom is calculated to run to the start of the next symbol or the end of the
section.

Other sections types can be implicitly cleaved.  For instance c-string literals
or unwind info (e.g. .eh_frame) can be cleaved by having the Reader look at 
the content of the section.  It is important to cleave sections into Atoms
to remove false dependencies.  For instance the .eh_frame section often
has no symbols, but contains "pointers" to the functions for which it 
has unwind info.  If the .eh_frame section was not cleaved (but left as one
big Atom), there would always be a reference (from the eh_frame Atom) to 
each function.  So the linker would be unable to coalesce or dead stripped 
away the function atoms. 

The lld Atom model also requires that a reference to an undefined symbol be
modeled as a Reference to an UndefinedAtom.  So the Reader also needs to 
create an UndefinedAtom for each undefined symbol in the object file.

Once all Atoms have been created, the second step is to create References 
(recall that Atoms are "nodes" and References are "edges").  Most References
are created by looking at the "relocation records" in the object file.  If 
a function contains a call to "malloc", there is usually a relocation record
specifying the address in the section and the symbol table index.  Your
Reader will need to convert the address to an Atom and offset and the symbol
table index into a target Atom.  If "malloc" is not defined in the object file,
the target Atom of the Reference will be an UndefinedAtom.  


Performance
-----------
Once you have the above working to parse an object file into Atoms and 
References, you'll want to look at performance.  Some techniques that can
help performance are:

* Use llvm::BumpPtrAllocator or pre-allocate one big vector<Reference> and then  
  just have each atom point to its subrange of References in that vector.  
  This can be faster that allocating each Reference as separate object.
* Pre-scan the symbol table and determine how many atoms are in each section
  then allocate space for all the Atom objects at once.  
* Don't copy symbol names or section content to each Atom, instead use
  StringRef and ArrayRef in each Atom to point to its name and content in the  
  MemoryBuffer. 


Testing
-------

We are still working on infrastructure to test Readers.  The issue is that
you don't want to check in binary files to the test suite. And the tools 
for creating your object file from assembly source may not be available on
every OS.  

We are investigating a way to use yaml to describe the section, symbols,
and content of a file.  Then have some code which will write out an object
file from that yaml description.  

Once that is in place, you can write test cases that contain section/symbols 
yaml and is run through the linker to produce Atom/References based yaml which  
is then run through FileCheck to verify the Atoms and References are as 
expected.